[clang] [Clang] raise extension warning for unknown namespaced attributes (PR #120925)

[Aaron Ballman via cfe-commits]

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@@ -822,8 +822,9 @@ def NSobjectAttribute : DiagGroup<"NSObject-attribute">;
 def NSConsumedMismatch : DiagGroup<"nsconsumed-mismatch">;
 def NSReturnsMismatch : DiagGroup<"nsreturns-mismatch">;
 
+def UnknownAttributeNamespace : DiagGroup<"unknown-attribute-namespace">;
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AaronBallman wrote:

Do we want to have a slightly different diagnostic wording for this case? e.g., `unknown attribute namespace '%0'; attribute '%0::%1' ignored`?

That way it's more clear that we don't know any attributes from that namespace?

Also, given that it's `-Wunknown-attributes` (plural) should this be `-Wunknown-attribute-namespaces`? I don't have a strong opinion, just caught my attention that one was plural and the other wasn't.

https://github.com/llvm/llvm-project/pull/120925